1. Field of the Invention
This invention relates to radiation sensitive materials and their use in pattern forming and transferring processes. More particularly, it relates to use of radiation sensitive materials based upon mixtures of polyoxometalates and organic compounds.
2. Description of the Related Art
Radiation sensitive materials, i.e. materials which change properties when exposed to some form of radiation, have many industrial applications. For example, such materials are typically used to make photographic and x-ray imaging film. Such film typically consists of a layer of radiation sensitive material deposited on a transparent sheet of substrate. A pattern or image may be formed in the film by exposing regions of the radiation sensitive layer to varying doses of radiation.
Radiation sensitive materials that have certain characteristics can be used in pattern transfer processes, wherein a pattern or image is first formed in a layer of the radiation sensitive material, and then the pattern is transferred to a second material. This type of technique is used, for example, in microlithography, photoengraving, and the manufacture of masters for optical recording.
With the increasing importance of computer technology in society over recent years, microlithographic techniques for making integrated circuits on semiconductor chips have received a great deal of attention. In microlithographic processes, radiation sensitive materials are used as resists in masking patterns to be etched into an underlying layer on or in a silicon wafer. In such processes, a layer of radiation sensitive material is typically coated on the wafer. A pattern is formed in this layer by exposing selected areas of the layer to radiation, e.g. ultraviolet light. Exposure to such radiation acts to alter the radiation sensitive material in such a way that either the exposed or unexposed areas of the photoresist layer can be removed (i.e. developed), e.g. with a solvent or gas plasma with or without an electric field. Thus, windows can be opened in the radiation sensitive layer to expose selected areas of the underlying silicon substrate. Subsequently, the exposed areas of the silicon substrate can be etched, while the remaining areas of the radiation sensitive layer act to resist the etch process and thus protect the underlying areas of substrate. This effectively transfers the pattern which was formed in the radiation sensitive material to the silicon substrate.
In such a process, since the developed layer of radiation sensitive material resists the etch process during the pattern transfer step, it is commonly called a "resist". If ultraviolet (UV) or visible light is used to expose the etch resistant material, that material is typically referred to as a "photoresist".
Radiation patterns are typically projected onto an unexposed resist layer through a mask or reticle. One way is to place the mask over the layer, some areas of the mask being radiation attenuating and others being transparent. Radiation can then be directed through the mask to the resist layer, and the resist areas underlying the transparent regions of the mask will be exposed. In this technique, the pattern on the resist layer will be the same size as on the mask. The mask may be placed directly onto the resist layer, a technique called contact lithography, but the more frequently used technique is to provide a small gap between the mask and the resist layer. The latter technique, commonly referred to as proximity lithography, minimizes damage to the mask since physical contact with the resist layer is avoided.
The image formed in the resist layer can be reduced in size by using projection lithography. Problems can arise in connection with this technique if the thickness of the resist layer approaches the depth of focus of the equipment used. For example, if the resist layer required to protect existing topography on a substrate is about 1 micron, and the depth of focus is about 1 micron, the plane of focus must be precisely located in the middle of the resist layer, or the pattern projected into the upper or lower thickness of the resist layer may be out of focus, resulting in a blurred pattern.
To address this problem, bilayer resist systems are sometimes used. In this type of system, a first layer, often called a "planarizing layer" since it is used to planarize existing topography on the substrate, is coated onto the wafer. Then, a film of radiation sensitive material is coated onto the planarizing layer. The pattern is then projected onto the photosensitive layer and developed. The planarizing layer is then etched, followed by etching the wafer.
The continuing trend in the manufacture of semiconductor devices is toward smaller and smaller device sizes, allowing a higher density of devices (e.g. resistors, transistors, capacitors) on a single chip. Currently, the minimum line width used in production microlithography is around 0.8 micron. The minimum feature size that can be achieved in photolithography is often limited by exposing wavelength. Within the next decade, minimum feature sizes of less than 0.5 micron have been projected to become the norm. Existing near UV exposure systems have typically been unable to provide the resolution needed to print these size features. As a result, there is great interest in exposure sources having shorter wavelength and which can be implemented into practical exposure tools. Recognized as the most promising of these are the deep UV excimer laser (KrF operating at 248 nm), x-ray (synchrotron generated with wavelength approximately 10 angstroms), and electron beam (e-beam) sources. Ion beams may also be a viable alternative.
Because of their shorter wavelength, the x-ray and e-beam tools are in principle capable of considerably higher resolution. The resolution of excimer laser-based lithographic tools is also adequate for most applications and the capital costs associated with those systems are potentially lower. It is believed that all three technologies will play an increasing role in the manufacture of semiconductor devices.
Along with the need for higher resolution exposure tools, there is a need for photoresist materials that are suitable for the shorter wavelengths. Current resists often show poor sensitivity or unacceptably high absorbance. Thus, there is interest in devising new classes of photoresists.
Most of the microlithographic resist technology of the U.S. today is organic polymer based. A major advantage of organic over most inorganic resists is that they can be dissolved in solvents and spun on wafers to form defect-free films. This advantage in industrial applications usually outweighs the superior resolution, contrast, resistance to reactive ion etching (RIE), defocus tolerance, and over/under exposure tolerance of the sputtered, or evaporated Ag.sub.2 Se/Ge.sub.0.15 Se.sub.0.85 inorganic resists.
Recently, researchers at Hitachi have developed a purely inorganic photoresist based on peroxopolyniobotungstic acid chemistry (T. Kudo, et al., J. Electrochem. Soc. Vol. 134, at 2607 (1987)). Using the resist in a bilayer scheme, they reported resolving 0.3 micron features reliably. The resist was reported to show deep UV, x-ray, and e-beam sensitivity. However, the peroxopolyniobotungstic acid resist has been found to be slower by about a factor of five than state-of-the art organic resists. The specific D.sub.0.5 values are 150 mJ/cm.sup.2 for deep UV light; 10 microcoulombs/cm.sup.2 for 30 kV e-beam exposure; and 120 mJ/cm.sup.2 for x-rays (MoL). Speed of exposure is, however, important, because of the high capital investment in exposure tools. Thus, the relatively low sensitivity is a significant disadvantage.
Several characteristics are important in relation to how well a material will perform as a photoresist in microlithography:
First, the material should be soluble in a suitable solvent, so that it can be spin-coated onto a substrate to form a film of uniform thickness. Spin-coating is the predominant industrial method for applying layers of photoresist materials on silicon and other wafers in process. Environmentally safe and economical solvents are of course preferred.
Second, the solution of the resist should be of such consistency that upon spin-coating onto a substrate, the film will be substantially defect-free, without pinholes or other irregularities, and be of substantially uniform thickness.
Third, the resist should provide sufficiently high resolution so that a well-defined pattern having small features can be formed in the film by irradiation followed by developing.
Fourth, sensitivity (i.e. the dose of radiation required for exposure) of the material to radiation is an important feature. The material should have high sensitivity so that it can be exposed quickly and economically.
Fifth, it should maintain the pattern in a well-defined manner upon developing (i.e. removal of exposed or unexposed areas) of the pattern in the photoresist layer. Preferably, the developing process used should involve only environmentally safe and economical developers.
Sixth, it should provide resistance to etching processes, particularly anisotropic etching processes, so that the pattern in the photoresist film can be precisely transferred to the underlying substrate.
Next, the photoresist material should have the ability to be easily stripped after transfer of the pattern to underlying layers.
Other characteristics which may be important involve stability and repair. The resist solution should preferably have adequate shelf life, and the image formed in the film should be stable after exposure. Also, the film should be relatively easy to remove at all processing stages in case of error in processing.
There is a continuing need to develop improved photoresist materials and systems which exhibit these and other characteristics. Improved radiation sensitive materials and processes would also be useful in other contexts which involve transferring patterns or images from one medium to another. It is accordingly an object of the present invention to provide materials and pattern transfer processes which exhibit at least some of these characteristics, and address at least some of the shortcomings experienced by prior art systems.
As indicated above, the invention disclosed herein relates to radiation sensitive materials based upon mixtures of organic compounds and polyoxometalates. The term "polyoxometalate" refers to materials containing polyoxoanions, and is used herein to include both isopolyoxoanions and heteropolyoxoanions, and derivatives of these materials. (Herein the term "isopolyoxoanion" is used synonymously with "isopolyanion" and "heteropolyoxoanion" synonymously with "heteropolyanion". Also, when the term "polyoxometalate" is used herein, it should be understood that such material may be provided in either salt or acid forms, the salt or acid providing isopolyoxoanions or heteropolyoxoanions in solution.
Isopoly and heteropolyoxoanions can be represented by the general formulas:
______________________________________ [M.sub.m O.sub.n ].sup.p- isopolyanion [A.sub.a M.sub.m O.sub.n ].sup.q- a .ltoreq. m heteropolyanion ______________________________________
Where M, called the addenda or peripheral atoms or simply the metal atoms, can come from one or more of the following metals: tungsten, molybdenum, vanadium, niobium, or tantalum. These atoms are generally in their higher oxidation states when they form polyoxometalates. The atoms A are called the heteroatoms and can come from almost any group of the periodic table.
By way of background, the molecular structures of polyoxometalates are generally based on distorted octahedra that combine by sharing corners, edges, or faces. The octahedra have the general formula [MO.sub.6 ].sup.R- and have the metal in their interior, with the oxygens in the corners of the octahedra. When they are combined, four or five of the oxygens in a single octahedron in effect act as bridges between metal atoms of different octahedra. The remaining two or one oxygens of each octahedron are not bridging oxygens. These oxygens are bonded with single metal atoms forming short M--O bonds. These are multiple bonds because of significant pi bonding between metal d orbitals and oxygen p orbitals of suitable symmetry. These are called external oxygens since they are oriented toward the exterior of the anion. In general, these oxygens are not basic and cannot form oxygen bridges. As a result of this, the polyoxoanion structures that form are closed, and so the polymerization does not extend indefinitely. This allows the formation of well characterized polyoxoanions.
Polyoxometalates with structures based not on octahedra but on other polyhedra such as square pyramids exist, but are relatively rare (e.g. [V.sub.18 O.sub.42 ].sup.12-). But even in these cases, external oxygens must generally be present to prevent further polymerization. In addition, there must generally not be more stable alternate structures with fewer atoms, because the polyoxoanions based on other polyhedra would decompose into these.
There seem to be two basic characteristics that limit the elements that can form polyoxometalates. First the metals that form polyoxometalates should have the correct size (cationic radius) in order to be six coordinated and so to exist in an octahedral oxide environment. In addition, they should be able to form pi bonds with the external oxygens, so they should be good p-pi acceptors. These are the properties that make the metals tungsten, molybdenum, niobium, vanadium, and tantalum in their higher oxidation states capable of forming polyoxometalates. Other elements may not have these properties sufficiently to form discrete and stable polyoxometalates.
The known structures of polyoxometalates are numerous, but some of them are more common since they are more stable in aqueous solution. Generally, when a simple salt, e.g. Na.sub.2 WO.sub.4 is dissolved in aqueous solution, a series of hydrolytic processes begin which lead to the formation of different isopoly- or heteropolyoxoanions, depending on the acidity of the solution, the presence of other species and their relative concentrations, the total ionic strength, and the processing conditions.
The most common isopolyoxoanions of W in aqueous solution are the following: [W.sub.7 O.sub.24 ].sup.6-, [W.sub.12 O.sub.42 H.sub.2 ].sup.10-, [W.sub.12 O.sub.42 ].sup.10-, [W.sub.10 O.sub.32 ].sup.4-, and [(H.sub.2)W.sub.12 O.sub.40 ].sup.6-, (although the last example can also be considered a heteropolyoxoanion with H as the heteroatom.) These are easily converted from one into the other by for example, changing the acidity of the solution. Other isopolyoxoanions may be more common in non-aqueous media (e.g. [W.sub.6 O.sub.19 ].sup.2-).
Many elements can act as heteroatoms in the formation of heteropolyoxoanions. As a result, the number of possible heteropolyoxoanions is in general much greater than in the case of isopolyanions. Nevertheless, there are some heteropolyoxoanion structures that are more common and stable. These are usually the more symmetric structures. One example is the Keggin structure in which there is a ratio of 1:12 between the heteroatom and the metal atoms. Polyoxometalates having this structure are formed with molybdenum and tungsten. There are related structures which are also considered fairly stable, e.g. structures in which a different metal replaces some of the metal atoms, such as replacing some tungsten atoms with vanadium atoms as in [PVW.sub.11 O.sub.40 ].sup.4-.
In the Keggin structure the heteroatom is generally located in the center of the polyanion. It is tetrahedrally coordinated. Examples of elements that are able to act as heteroatoms in Keggin structures are P, Si, B, Fe, and Co. Changing the heteroatom can cause subtle changes in the polyoxoanion. For example, exchanging P for Si changes the charge of the anion, shifts the absorption maximum in the UV, and shifts the half-wave reduction potential. However, basic chemical properties, such as the ability to photooxidize organic compounds, are similar.
Other heteroatoms may not be compatible with Keggin type structures. For example, Mo forms a Keggin structure with P as heteratom [PMo.sub.12 O.sub.40 ].sup.3-, but forms a quite different structure with Ce as the heteroatom [CeMo.sub.12 O.sub.42 ].sup.8-. This last structure is different even though it has the same ratio (1:12) of hetero- to metal atoms. The octahedra that combine to form the Keggin structure contain only one non-bridging (external) oxygen whereas the octahedra which combine to give [CeMo.sub.12 O.sub.42 ].sup.8- contain two external oxygens.
The same heteroatom can also form more than one heteropolyoxoanion with the same metal. For example, in the case of P and W, anions are known with P:W ratios of 2:5, 1:9, 1:11, 1:12, 2:17, 2:18, 2:19, and 2:21. Some of them are very closely related; for example, [PW.sub.12 O.sub.40 ].sup.3- and [PW.sub.11 O.sub.39 ].sup.7- can be converted from one to the other by changing the acidity of the solution as in the case of the isopolytungstates.
As can be seen, many structures are possible, but in almost every case there are common characteristics that make the polyoxometalates a well defined class of chemical compounds with comparable chemical properties. The same is true for their derivatives, that is, there can be partial substitution either of the bridging oxygens with for example peroxo groups, or of the metal with other metals, e.g., replacing some of the Mo with Ti. In this last example, the new metal (Ti) can be found with even an organic ligand (e.g. n--C.sub.5 H.sub.5) in the place of an external oxygen. The whole polyoxometalate can also act as a ligand of another metal, for example [Mn.sup.IV (Nb.sub.6 O.sub.19).sub.2 ].sup.12-. These compounds are also treated as polyoxometalates herein, since they still contain polyhedra of metals (e.g. W, Mo, Nb, Ta, and V) with oxygens as bridging or external atoms.
The present invention relates to the photochemistry of polyoxometalates with organic compounds. This photochemistry has been studied systematically for the last 10-12 years and is well known in the literature. These studies have shown that a series of polyoxometalates can be photoreduced in the presence of a variety of organic compounds. Polyoxometalates that have been studied extensively include 1:12 and 2:18 heteropolymolybdates and the corresponding heteropolytungstates, mixed heteropolyanions such as [PMo.sub.10 V.sub.2 O.sub.40 ].sup.5- and [PW.sub.10 V.sub.2 O.sub.40 ].sup.5-, and isopolyoxoanions such as [W.sub.10 O.sub.32 ].sup.4-, [Mo.sub.7 O.sub.24 ].sup.6-, and [V.sub.10 O.sub.28 ].sup.6-. These compounds have mainly been studied because of their utility in photocatalytic oxidation of organics, and photoproduction of hydrogen.